An ultraviolet curable resin is a photosensitive material that cures when irradiated with ultraviolet rays. A technique for curing such an ultraviolet curable resin is referred to as a UV curing technique. According to the UV curing technique, for example, an ultraviolet curable resin applied to an object to be processed can be cured for dry processing. An ultraviolet curable resin interposed between two objects to be processed can be cured for adhesion processing of the two objects to be processed.
Processing using the UV curing technique is power saving processing because an ultraviolet irradiation time needed to cure the ultraviolet curable resin is several seconds to several minutes depending on the size of the object(s) to be processed. Since the ultraviolet curable resin can be cured only by the irradiation with ultraviolet rays, the processing is low-temperature processing and the object(s) to be processed can thus be plastics lacking heat resistance, precise electronic parts and the like.
The UV curing technique having the foregoing characteristics is used in various fields.
As specific examples of the use of the UV curing technique, may be mentioned adhesion processing of an electronic part or an optical part by using an adhesive made of an ultraviolet curable resin, drying processing (fixing processing) of printing ink made of an ultraviolet curable resin and drying processing of a coloring paint made of an ultraviolet curable resin on automobile parts, electrical appliances, building materials, plastic parts, etc. (for example, see Patent Literature 1). In the field of paints, the UV curing technique is applied to the drying processing of not only coloring paints but also a luster coating material (hard coat material) on a vehicle body of an automobile, a motorcycle and the like.
An ultraviolet curable paint made of an ultraviolet curable resin contains a photopolymerization initiator, a photopolymerizable monomer and a photopolymerizable oligomer as main components contributing to curing (ultraviolet curing).
For example, the ultraviolet curing of the ultraviolet curable paint is performed through the following curing reaction (ultraviolet curing reaction).
If the ultraviolet curable paint is irradiated with ultraviolet rays, the photopolymerization initiator initially absorbs the ultraviolet rays. The photopolymerization initiator is thereby activated to generate radicals or ions. The generated radicals or ions react (cause a polymerization reaction) with the photopolymerizable monomer and the photopolymerizable oligomer for curing.
The UV curing technique uses a high-pressure mercury lamp or a metal halide lamp as an ultraviolet radiation unit.
The high-pressure mercury lamp includes a quartz glass light emitting tube filled with high-purity mercury and a rare gas. The high-pressure mercury lamp radiates ultraviolet rays having a main wavelength of 365 nm, and wavelengths of 254 nm, 303 nm and 313 nm with high efficiency.
The metal halide lamp includes a light emitting tube filled with mercury and a metal halide. The metal halide lamp radiates light in a range as wide as 200 nm to 400 nm in wavelength, specifically ultraviolet rays and visible rays. The output of long-wavelength ultraviolet rays from the metal halide lamp is higher than that from the high-pressure mercury lamp.
If the ultraviolet curable paint is used as a luster coating material, a photopolymerizable monomer and a photopolymerizable oligomer that form a colorless transparent polymer (cured article) are used as those constituting the ultraviolet curable paint. The purpose is so that the base color of the object to be processed (specifically, the color of a coloring layer formed on the surface of the base substance of the object to be processed by using a coloring paint, i.e., the color of the underlayer) can be visually observed through the luster coating layer (a paint cured layer of the ultraviolet curable paint).
In fact, the object to be processed on which the luster coating layer is formed of the ultraviolet curable paint has the problem that the base color of the object to be processed (for example, the color of the coloring layer constituting the underlayer) is not visually observable but looks yellowed.
To be more specific, the photopolymerization initiator contained in the ultraviolet curable paint is certainly sensitive to light in an ultraviolet region. Many of such a photopolymerization initiator are also sensitive to light in a visual ray region (for example, see FIG. 2 of Patent Literature 2). That is, many photopolymerization initiators used in ultraviolet curable paints have a sensitivity wavelength region that covers shorter wavelength regions (specifically, indigo to blue regions) of the visible ray region on the longer wavelength side. FIG. 2 of Patent Literature 2 illustrates absorption spectrum distributions of certain types of photopolymerization initiators. Such an ultraviolet curable paints containing a photopolymerization initiator sensitive to light in the visible ray region has a characteristic that the photopolymerization initiator absorbs some of the light in the visible ray region even after cured. For example, if the luster coating layer is formed on an object to be processed on which a coloring layer is formed by using a white paint, the color of the object to be processed looks yellowish. In other words, the color of the underlayer of the object to be processed (white of the coloring layer) is not visually observable. Such a problem is not specific to the case where the base color of the object to be processed is white, but can also occur if the base color of the object to be processed is other than white.
To avoid such a problem, an ultraviolet curable paint containing a photopolymerization initiator not sensitive to the light in the visible ray region may be used as the luster coating material. More specifically, ultraviolet curable paints containing photopolymerization initiators having absorption spectrum distributions (sensitivity wavelength characteristics) such as illustrated by curves (A) to (C) of FIG. 5 may be used.
If such an ultraviolet curable paint is used as the luster coating material, the formed luster coating layer is prevented from absorbing some of the light in the visible ray region. The base color of the object to be processed (for example, the color of a coloring layer constituting the underlayer) on which the luster coating layer is stacked can thus be visually observed.
However, the use of an ultraviolet curable paint having such a sensitivity wavelength characteristic can cause a problem that the curing of the ultraviolet curable paint may become insufficient.
More specifically, as evident from FIG. 5, the high-pressure mercury lamp and the metal halide lamp conventionally used as the ultraviolet radiation unit have a characteristic that the light intensity is high in a region of not less than 300 nm in wavelength, greater than 350 nm in particular. In FIG. 5, a curve (a) indicates an emission spectrum distribution of the high-pressure mercury lamp. A curve (c) indicates an emission spectrum distribution of the metal halide lamp. The high-pressure mercury lamp has the highest light intensity at a peak wavelength of 365 nm. The metal halide lamp has the highest light intensity at a peak wavelength of longer than 360 nm.
As is evident from the curves (A) to (C) of FIG. 5, the photopolymerization initiators not sensitive to the light in the visible ray region have a sensitive wavelength characteristic (absorption spectrum distribution) that the sensitivity to light in a region of greater than 350 nm in wavelength is low.
The high-pressure mercury lamp and the metal halide lamp both radiate light in a region of not greater than 350 nm in wavelength where the photopolymerization initiators not sensitive to the light in the visible ray region have high sensitivity. However, the light intensity is small as compared to that of the light in the region of greater than 350 nm in wavelength.
If an ultraviolet curable paint containing a photopolymerization initiator having a sensitivity wavelength characteristic such as illustrated by the curves (A) to (C) of FIG. 5 is used to form the luster coating layer on the object to be processed, the ultraviolet curing reaction of the ultraviolet curable paint becomes slow. In some cases, the curing can be insufficient.